Cucurbit yellow stunting disorder virus (CYSDV) is a closterovirus transmitted in nature by the whitefly Bemisia tabaci. CYSDV was first detected in 1982 in the United Arab Emirates, and since then, it has been found in Spain, Portugal Morocco, Lebanon and North America extensively affecting cucurbit crops. CYSDV induces interveinal chlorotic spots in mature leaves which may enlarge and eventually fuse together producing yellowing of the entire leaf except for the veins that remain green. Yellowing symptoms are accompanied by substantial reduction in fruit yield and quality and, therefore, the virus has a high economic importance.
The control of CYSDV is currently based on chemical treatments against its vector and preventive cultural practices, both with limited success. The use of genetically resistant cultivars is a good option for CYSDV control.
Resistant accessions of melon (Cucumis melo), such as accession C-105 (TGR-1551), have recently been found (López-Sesé et al., 2000). In principal, genetic material from such accessions that comprises the genetic information responsible for the CYSDV resistance could be introgressed into commercial cultivars. However these attempts have hitherto been unsuccessful for unknown reasons. Therefore no resistant melon cultivar is at present commercially available. It is believed that the practical problem of developing resistant cultivars is hampered by two factors. Firstly, it is not known whether such specific accession as C-105 provides the best source of resistance. If the result is a cultivar exhibiting partial resistance, the result of the market introduction of such a cultivar may lead to the development of resistance-breaking viral strains. Furthermore, the possibility of stably fixing the resistance trait in the genome of the target plant remains to be determined. Thus, choosing a particular source of resistance does not guarantee success and may even hold various risks. In addition, the introgression itself involves a substantial breeding effort, and includes the development and performance of bioassays to follow resistant offspring plants. In effect, the development of a resistant cultivar is commercially costly undertaking and any program may be early abandoned when results fail to precipitate.
In order to reduce the uncertainties and work involved in developing a resistant cultivar, it would be beneficial to have a simple genomic marker for the resistance trait. Such a marker could then be used in marker assisted selection (MAS) procedures as part of a dedicated breeding program. Whether such a marker can be found is partly determined by genomic structure of the resistance trait. If the trait is multi-genic, it is not likely that a single marker is found that may reliably be used in MAS procedures. Moreover, the development of the marker(s) themselves then mounts to a significant undertaking, possibly overshadowing the costs and time involved in a straightforward breeding program.
However, once a suitable resistance source is identified and a marker is developed, the new resistant plants can be easily traced, which increases their commercial value.